The present invention relates to a fluorescent lamp lighting device, and more particularly, to a starting switch circuit using a semiconductor switching element for a fluorescent lamp which includes warm-up type electrodes.
A conventional starting switch circuit for a fluorescent lamp lighting device was primarily a glow starter. However, a glow starter has problems such as a long time needed for starting and a short lifetime. Although starting switch circuits using a semiconductor switching element have been developed, such starting switch circuits are expensive and met by only a limited demand. Therefore, development of an inexpensive starting switch circuit has been demanded. In a prior art, use of a transistor as a semiconductor switching element as shown in FIG. 6 is proposed (Japanese Patent Application Laid-Open Gazette No. 3-252096).
A circuit of a conventional fluorescent lamp lighting device shown in FIG. 6 comprises an a.c. power source 1, a ballast 2, a fluorescent lamp 3 including a pair of warm-up type electrodes 4, 5, a noise suppression capacitor 6, and a starting switch circuit 7. The circuit configuration of the fluorescent lamp lighting device is as follows. One end of the ballast 2 is connected to the a.c. power source 1. The other end of the ballast 2 is connected to a power source side terminal of the electrode 4 of the fluorescent lamp 3. The starting switch circuit 7 and the noise suppression capacitor 6 are connected between starting switch circuit side terminals of the electrodes 4, 5 of the fluorescent lamp 3.
Intra-circuit configuration of the starting switch circuit 7 which is connected between the starting switch circuit side terminals of the electrodes 4, 5 of the fluorescent lamp 3 is as follows. A first rectifying element 8, a resistor 30 and the collector and the emitter of a transistor 31 are connected in series between the starting switch circuit side terminals of the electrodes 4, 5 of the fluorescent lamp 3. A resistor 32 is connected between a connection point between the first rectifying element 8 and the resistor 30 and the base of the transistor 31. A timer circuit, which is formed by connecting a resistor 37 and a capacitor 36 in series to each other, is connected between the first rectifying element 8 and the starting switch circuit side terminal of the electrode 5. A thyrister 33 which is controlled by the timer circuit is connected between the base and the emitter of the transistor 31. A series circuit comprising a resistor 34 and a Zener diode 35 is connected between a connection point between the capacitor 36 and the resistor 37 and the gate of the thyrister 33.
Next, an operation of the conventional fluorescent lamp lighting device will be described.
The a.c. power source 1 is turned on. During a positive cycle of a power source voltage upon turning on of the a.c. power source 1, a base current is supplied to the base of the transistor 31 through the first rectifying element 8 and the resistor 32. Since the base current is supplied to the base of the transistor 31, a collector current flows between the collector and the emitter of the transistor 31 through the first rectifying element 8 and the resistor 30. A half-wave warm-up current flows in this manner, and therefore, the electrodes 4, 5 of the fluorescent lamp 3 are warmed up. At the same time, a reverse voltage is applied across the Zener diode 35. When the reverse voltage reaches a predetermined value (hereinafter "Zener voltage"), a current starts flowing in a reverse direction. As used herein, the phrase "becomes to ON state" refers to a device conducting, or being in an "ON" state. A charge accumulated in the capacitor 36 flows to the gate of the thyrister 33 through the Zener diode 35 and the resistor 34, so that the thyrister 33 becomes to ON state, and a current accordingly flows between the anode and the cathode of the thyrister 33. As used herein, the phrase "becomes to OFF state" refers to a device being non-conductive, or being in an "OFF" state. Therefore, the current which has been flowing to the base of the transistor 31 stops flowing, whereby the transistor 31 becomes to OFF state. As a result, a kick voltage due to an inductance of the ballast 2 is generated, and the fluorescent lamp 3 is ignited.
In the starting switch circuit which is used for the conventional fluorescent lamp lighting device, an operation of the timer circuit is determined by the sum of a voltage between the collector and the emitter of the transistor 31 and a voltage across the resistor 30. The voltage between the collector and the emitter of the transistor 31 and the voltage across the resistor 30 are influenced by the warm-up current which flows between the collector and the emitter of the transistor 31, and an operation time of the timer circuit is also changed accordingly.
In the case of a low power fluorescent lamp lighting device using the ballast 2 which has a large inductance, the operation time of the timer circuit is long since the warm-up current is small. In the worse situation, the voltage charged up the capacitor 36 of the timer circuit stabilizes at a low voltage. If this occurs, a current does not flow to the gate of the thyrister 33 and the warm-up current keeps flowing continuously. This leads to heating up of the ballast 2 and blackening of the both ends of the fluorescent lamp 3, which in turn makes it impossible to ignite the fluorescent lamp 3. Further, the voltage of the a.c. power source 1 decreases instantaneously during burning of the fluorescent lamp 3. And therefore, upon recovering of the voltage after the fluorescent lamp 3 went off, the thyrister 33 remains in ON state because of the gate current from the a.c. power source 1 and the capacitor 36. The operation, as mentioned above, is not executed and the fluorescent lamp 3 remains off since the transistor 31 remains in OFF state, and hence, it is impossible to ignite the fluorescent lamp 3 unless a switch of the a.c. power source 1 is operated once again.